(a) Field of the Invention
The present invention relates to field emission displays (FEDs). More particularly, the present invention relates to FEDs that include electron emission sources made of carbon-based material.
(b) Description of the Related Art
In modern FEDs, a thick-layer process such as screen printing is used to form electron emission sources (i.e., emitters) in a flat configuration utilizing a carbon-based material that emits electrons at low voltage driving conditions (e.g., 10–100V).
Carbon-based materials suitable for forming the emitters include graphite, diamond, diamond-like carbon, and carbon nanotubes (CNTs). Among these, carbon nanotubes appear to be very promising for use as emitters because of their extremely minute tips with a radius of curvature of approximately tens to several tens of nanometers, and because carbon nanotubes are able to emit electrons in low electric field conditions of about 1–10V/μm.
U.S. Pat. Nos. 6,062,931 and 6,097,138 disclose cold cathode field emission displays that are related to this area of FEDs using CNT technology.
In the case where the FEDs employ a triode structure having cathode electrodes, an anode electrode, and gate electrodes, cathode electrodes, an insulation layer, and gate electrodes are formed on a rear substrate in this order, holes are formed in the gate electrodes and the insulation layer to expose the cathode electrodes, then emitters are formed on exposed surfaces of the cathode electrodes. Also, an anode electrode and phosphor layers are formed on a front substrate.
However, with such a structure, when providing emitter material on the surfaces of the cathode electrodes exposed through the holes, the emitter material may extend between the cathode electrodes and gate electrodes to form short circuits between these two elements. Further, with the conventional triode structure, when the electrons emitted from the emitters are formed into electron beams and travel toward the phosphor layers, a diverging force of the electron beams is increased by influence of a positive voltage applied to the gate electrodes such that the electron beams disperse.
To remedy these problems, with reference to FIG. 26, there is disclosed an FED in which gate electrodes 3 are first formed on rear substrate 1, and after insulation layer 5 is formed over gate electrodes 3, cathode electrodes 7 and emitters 9 are formed on insulation layer 5. The formation of a short circuit between cathode electrodes 7 and gate electrodes 3 by emitter material is avoided with this configuration. Also, since emitters 9 are formed lastly (i.e., on an outermost layer of rear substrate 1), emitters 9 are easily formed on cathode electrodes 7.
In the FED of FIG. 26, emitters 9 are typically formed along one long edge of cathode electrodes 7, and the electric field induced from gate electrodes 3 surround emitters 9 to realize field emission. However, since cathode electrodes 7 are generally made having a significant width to ensure good conductivity, significant influence of the electric fields causing field emission is limited only to the edges of emitters 9.
As a result, compared to the FED using the conventional triode structure, the electric field intensity around emitters 9 is significantly lower, and the drive voltage and power consumption required for electron emission is high since the area of field emission is limited. Also, because of this small area of field emission, the amount of electrons that are emitted is small such that there are limitations in increasing screen brightness.
Further, in the FED of FIG. 26, if a distance between cathode electrodes 7 exceeds a predetermined amount (for example, more than one-third of a gate electrode pitch), a neighboring effect, in which variations in the electric field intensities in the vicinity of emitters 9 occur by a data voltage applied to gate electrodes 3 and by a data voltage of adjacent gate electrodes 3.
With respect to a specific emitter 9 forming one pixel, the neighboring effect refers to the phenomenon in which if a data voltage is applied to gate electrode 3 of an adjacent pixel, the electric field around emitter 9 of this pixel is significantly strengthened such that emission current is increased, and if a data voltage is not applied to gate electrode 3 of an adjacent pixel, the electric field around emitter 9 of this pixel is weakened such that electron emission is decreased.
Therefore, if a data voltage is applied to a specific gate electrode 3, electron emission occurs not only from emitter 9 corresponding to this gate electrode 3 but also from adjacent emitters 9 such that phosphor layer 11 surrounding the intended phosphor layer 11 are illuminated, thereby reducing color purity. In addition, although brightness is maintained in the case where a white color is displayed on the screen, if colors are displayed, these areas may become dark such that uneven brightness occurs in the picture.
The above problem may be minimized by reducing the distance between cathode electrodes 7. In one embodiment where the pitch of gate electrodes 3 is 320 μm, the neighboring effect disappears if the distance between cathode electrodes 7 is set at approximately 20 μm.
However, if cathode electrodes 7 are positioned too closely, the data voltage applied to cathode electrodes 7 is cut off by adjacent cathode electrodes 7 such that an increase in the electric field of corresponding emitters 9 is unable to be realized. Therefore, the control of field emission by gate electrodes 3 is not possible, thereby making matrix driving unattainable.
Further, in the FED described above, the electric fields are concentrated on the edges of emitters 9 so that electron emission occurs from only the edges of emitters 9. A characteristic of such edge emission is that the electron beams formed by the emission of the electrons from emitters 9 are unable to travel perpendicularly in a direction toward corresponding phosphor films 11, and instead travel by spreading parabolically in a predetermined arc. Hence, the electron beams emitted from emitters 9 land not only on phosphor layer 11 in the intended pixel, but on phosphor layers 11 of adjacent pixels to illuminate the same. Color purity is reduced as a result and precise images are unable to be realized.
In the conventional FED, picture brightness is proportional to the amount of electrons emitted from emitters 9 and the voltage applied to anode electrode 13. Since the anode current density per unit area of phosphor layers 11 is limited to a predetermined amount when considerations are made to the lifespan of phosphor layers 11, picture brightness is increased when an even higher voltage is applied to anode electrodes 13.
However, with the conventional structure of emitters 9 being opposed to anode electrode 13 with a significant distance therebetween, if an excessive voltage is applied to anode electrode 13 to increase brightness, the electric field between cathode electrodes 7 and anode electrode 13 is increased and it is possible for arcing to occur. This results in damage or heating of emitters 9 such that uniformity in the illumination of the screen is reduced and the lifespan of the emitters deteriorates.